Carbon fibers are used as reinforcing agents in the manufacture of a variety of products, often in the form of carbon fiber-reinforced polymers (CFRPs), which are composite materials including carbon fibers as reinforcing agents bound in a matrix, typically a matrix of a plastic composition. The reinforcing fibers used in CFRPs are often covered with a layer of material, referred to as fiber sizing material, to protect the fibers during handling, shipping, storage and manufacture operations and to provide enhanced compatibility and binding with materials of a plastic matrix of the CFRP. CFRPs are used in a variety of consumer and industrial products. A high cost of virgin carbon fibers of industrial or commercial grade limits utilization in a broader-range of end-user applications, including limiting broader use in automotive and transportation sectors where there is significant potential for expanded use. CFRPs represent a subset of fiber-reinforced composites in which the reinforcing fibers are bound in a matrix. Other fiber-reinforced composites may include reinforcing fibers of materials other than carbon. The discussion here concerning fiber-reinforced composites is presented primarily with reference to CFRPs.
There are a variety of intermediate forms of reinforcing fiber-containing products in which reinforcing fibers, and including carbon fibers, may be present in the industrial chain between initial manufacture of the reinforcing fibers and a fiber-reinforced composite in final form. For example, following manufacturing of the reinforcing fibers, the reinforcing fibers may be covered with a thin layer of another material or materials to protect the reinforcing fibers from damage or degradation during handling, shipping, storage and manufacture operations and/or for enhanced performance interaction with materials of, or precursors for, the matrix of the intended final fiber-reinforced composite product form. Such a layer is often referred to as “fiber sizing” or more simply as “sizing”, and a material of such a layer may be referred to as a fiber sizing material, or more simply as “sizing material”. Including such a fiber sizing layer is a common practice for many carbon fiber applications. The process of applying fiber sizing may be incorporated into an integrated manufacturing operation in which fiber sizing is applied to the virgin reinforcing fibers soon after they are formed in the manufacture operation, for example to provide immediate protection to the reinforcing fibers and in a form compatible with the final intended matrix for a CFRP.
Alternatively, the virgin reinforcing fibers may be stored and/or shipped in the virgin form for later processing to apply fiber sizing in a separate manufacture operation. Such reinforcing fibers following application of fiber sizing may be referred to as sized reinforcing fibers, and products with the sized reinforcing fibers prior to addition of material for a matrix for a fiber-reinforced composite may be referred to as sized reinforcing fiber products. Sized reinforcing fiber products may be in a product form, for example, of individual strands, tow (e.g., untwisted bundle), yarn (e.g., twisted bundle) or mat or sheet form (e.g., woven or nonwoven forms). Virgin fibers, without fiber sizing applied, may also be prepared into such product forms. Such product forms may be further processed into a preliminary CFRP form, in an integrated manufacturing operation or a separate manufacturing operation, to add material for a matrix, which may be in the form of the final matrix composition (e.g. thermoplastic matrix compositions) or preliminary matrix composition form (e.g., uncured thermoset resin). Such a preliminary CFRP form may typically have the reinforcing fibers in the same general geometric arrangement (e.g., tow, yarn, sheet or mat form) as in the form prior to adding material of the matrix. A preliminary fiber-reinforced composite with an uncured thermoset resin material for a matrix is often referred to as a prepreg. Such preliminary CFRPs may be used in a final product manufacturing operation in which the CFRP is shaped into and set in a desired final product form (e.g., with heating followed by cooling in the case of a thermoplastic matrix and with curing in the case of a thermoset matrix). Such a final product form may be referred to as a final fiber-reinforced composite, or final CFRP in the case of carbon fibers. Such sized reinforcing fiber products, preliminary fiber-reinforced composites and final fiber-reinforced composites are all examples of reinforcing fiber-containing products, which include the reinforcing fibers in combination with one or more other materials (e.g., fiber sizing material and/or preliminary or final matrix material).
Even with the high cost of virgin carbon fibers, a significant quantity of CFRPs and other reinforcing fiber-containing products, and the reinforcing fibers therein, end up as waste. It is common in CFRP applications for material trim and scrap waste to amount to about 30% or more of finished part weight. In addition, preliminary CFRPs (and especially with uncured thermoset resin) and sized reinforcing fiber products may have a limited acceptable shelf-life prior to further use in a manufacturing operation (e.g., to prepare a CFRP in the case of sized carbon fiber products or to prepare a final CFRP in the case of a preliminary CFRP). Often times, such sized reinforcing fiber products or preliminary CFRPs may expire prior to being utilized, and end up as manufacturing waste. Such manufacturing waste, whether in the form of material trim, scrap, or expired product, is often incinerated or sent to a landfill, resulting in additional waste disposal costs and significant lost raw material value.
Such manufacturing waste represents a possible resource for recycled carbon fibers. For example, significant attempts have been made, especially for trim and scrap waste and expired CFRPs, to recover carbon fibers for recycling. However, effectively freeing carbon fibers for recovery from CFRP matrix and/or from fiber sizing material has proven difficult, with a result being that recycle processing has tended to be expensive and/or to result in significant degradation of carbon fiber properties, significantly limiting utility of recycling as a source of carbon fibers for a range of possible applications. Moreover, as will be discussed in greater detail below, recycle processing has also tended to result in processed carbon fibers of a lesser or degraded form as compared to the feedstock for such processes. For instance, during recycling, fibers often are severed, tangled, or frayed, limiting the available forms for recycled carbon fiber composites.
One recycling technique involves subjecting waste materials to pyrolysis. This technique utilizes high temperatures to decompose polymeric matrix while attempting to leave the reinforcing fibers intact. The carbon fibers recovered from this processing often have a short fiber length with limited potential for reuse in many products. Also, pyrolysis, as a process option, has significant limitations with respect to intensive energy requirements, high processing costs, and potential for negative environmental impact due to emission of pyrolysis by-products.
Another type of recycling technique uses chemical agents to chemically react with and degrade, and break down the polymeric matrix (sometimes referred to as depolymerization) to degradation products that may be separated from the carbon fibers, such as by dissolution of the degradation products into a solvent. Such processes tend to be expensive and may also degrade carbon fiber properties.
In addition, while the foregoing techniques have generally been considered for use in recycling of trim and scrap waste that include discontinuous reinforcing fibers, certain sources of recyclable material include continuous fibers such as continuous prepreg sheets or continuous prepreg tow. These materials in the continuous form may provide advantages for use in manufacturing processes and/or in finished products produced using the continuous forms. For example, products manufactured with unidirectional fiber reinforced sheets or tow material may provide enhanced directionalized part performance. As such, recycling techniques applicable to discontinuous fibers, such as those resulting from trim and scrap waste, may require severing, tangling, or fraying fibers which results in degradation of the continuous form, thus degrading the resulting recycled fiber product.
A need exists for improved processes to recover carbon fibers for recycling in a manner that increases the range of applications in which recycled carbon fibers may be technically and economically suitable for use. Moreover, an approach that maintains a continuous form of carbon fibers from waste materials is needed.